1. Field of the Invention
This invention relates to drilling fluids utilized in oil well drilling operations. More particularly, the present invention relates to shale stabilizers, to drilling fluids comprising such stabilizers and to a method of use thereof to stabilize shale formations encountered during an oil well drilling operation.
More particularly, the present invention relates to water-soluble copolymers of an amide of a vinyl carboxylic acid and a hydroxy-containing ester of a vinyl carboxylic acid. These copolymers are effective as shale hydration inhibitors and can be added to a drilling fluid and circulated through a shale formation whereby shale sloughing during an oil well drilling operation is significantly reduced.
To remove cuttings produced during an oil well drilling operation, a drilling fluid is circulated down the drill pipe and up the annulus between the drill pipe and the formation whereby the cuttings are carried to the surface. The cuttings are then shaken out of the drilling fluid and the fluid is recirculated. In addition to removing cuttings, the drilling fluid also coats and lubricates the drill bit and stabilizes the formation.
In some areas, formations known as heaving or sloughing shales are penetrated during a well-drilling operation. These shales have a tendency to swell or crack upon contact with the drilling fluid whereupon the bore hole walls are rendered unstable and the heaving shale material, which makes up such walls, sloughs into the bore hole. Sloughing of shale material into the bore hole can cause the drill stem to become stuck and can enlarge the bore hole with the result that large subterranean cavities are formed. Furthermore, where sloughing occurs while the drilling bit is being changed at the surface, the bore hole fills up and must be cleared before drilling can proceed. In addition, the heaving shale material taken up into the drilling fluid adversely affects the viscosity characteristics of the drilling fluid to the point where the fluid must be chemically treated to reduce the viscosity thereof or it must be diluted followed by the addition of weighting material to maintain a given mud weight.
The present invention provides shale stabilizers which can be added to a drilling fluid to eliminate or significantly reduce shale sloughing. The present shale stabilizers are comprised of water-soluble copolymers of an amide of a vinyl carboxylic acid (vinyl carboxamide) and an ester of a vinyl carboxylic acid wherein the ester includes a hydroxy-containing organic radical (hydroxy-containing vinyl carboxylic ester). Preferred vinyl carboxamides include acrylamide and methacrylamide. Preferred hydroxy-containing vinyl carboxylic esters include hydroxyalkyl and polyhydroxyalkyl esters, and hydroxypoly(alkoxy) esters of acrylic or methacrylic acid.
2. Prior Art
A wide variety of additives have been utilized in drilling fluids to stabilize shale formations encountered during oil well drilling operations. For example, Weiss et al. U.S. Pat. No. 2,802,783 disclose saturating an aqueous drilling fluid with calcium hydroxide; Scott et al. U.S. Pat. No. 3,017,351 disclose incorporating into the drilling fluid an acetate salt of an amide of a fatty acid and a polyamine along with ammonium sulfate, oil and an acrylamide polymer hydrolyte; Blattel et al. U.S. Pat. No. 4,341,645 disclose salts of copolymers of acrylic acid and hydroxypropyl acrylate; and Dupre et al. U.S. Pat. No. 4,299,710 disclose the combination of an acid-containing polymer and a polysaccharide wherein the acid-containing polymer can be a copolymer of any one of a long list of saturated carboxylic acids, including acrylic acid or a half-amide of a dicarboxylic acid, with any one of a long list of vinyl monomers, including 2-hydroxyethyl and hydroxypropyl acrylates. These additives all require formation of salts in order to be utilized in aqueous-based drilling fluids.
Copolymers of vinyl carboxamides, such as acrylamide and methacrylamide, and hydroxyalkyl esters of vinyl carboxylic acids, such as 2-hydroxyethyl acrylamide, and oxyalkylated derivatives thereof, have been utilized in a wide variety of forms for various applications. For example, Dusek U.S. Pat. No. 3,984,841 discloses hydrophilic gels which are useful in a variety of medical applications and which are derived from free radical initiated copolymers of 2-hydroxyethyl methacrylate and an amide of acrylic or methacrylic acid wherein the weight percent of acrylamide based on the total weight of the monomers is from about 20 to 80 percent and that of methacrylamide is from 20 to 90 percent. Merriam U.S. Pat. No. 2,870,129 discloses copolymers of acrylic acid or acrylamide and alkoxyalkyl acrylates which form water vapor permeable resins. Such gels and resins are not suitable for use as additives for drilling fluids.
Similar copolymers have also been prepared and utilized in a variety of applications. See, for example, Shulz et al. U.S. Pat. Nos. 4,463,151; 4,463,152 and 4,546,784 and Maurer et al U.S. Pat. No. 4,579,926 which disclose free radical initiated copolymers of acrylamide and alkyl poly(etheroxy) acrylates effective as brine viscosifiers. These copolymers are not effective for inhibiting shale hydration; Lucas et al. U.S. Pat. Nos. 4,268,400 and 4,288,582 which disclose terpolymers of a monovalent alkali metal salt of acrylic acid, a hydroxyalkyl acrylate and acrylamide for use as filtration control additives in drilling fluids; and Morduchowitz U.S. Pat. No. 4,323,463 which discloses a terpolymer of acrylamide, acrylonitrile and poly(oxyethyl) acrylate which is useful as a water thickener in a secondary oil recovery process. These terpolymers require formation of a monovalent alkali metal salt of the acrylic acid to be useful in aqueous-based drilling fluids.